"Under earlier modes of production, people bartered by exchanging commodity (C) for commodity (C). Later, money (M) helped us mediate those transactions: we sold commodities in return for money that we used to purchase another good (C-M-C).
Capitalists, however, relate to exchange differently. They used money from commodity sales to invest in production in order to make more goods that they were then compelled by competition to sell at a profit (M-C-M'). Much of the resulting yield was thrown back into making more commodities in the 'restless never-ending process of profit-making.' If capitalists ever slowed down or showed any mercy, they would get swallowed up by their rivals.
But Marx, of course, didn't just see markets and commodities when examining capitalism. He saw the labor of millions ensnared by it. In all but the most primitive societies, labor beyond a subsistence level created a surplus, and any surplus was appropriated, consumed, or otherwise directed by elites.
Under feudalism, exploitation was obvious. If peasants had to give 40 percent of their yield to their lord, they knew that out of the fifty days they labored in the fields, they were only reaping the rewards of thirty.
Marx pointed out that similar dynamics existed under capitalism. Workers couldn't just be employed for the 'necessary working time' required to ensure their existence. Capitalists kept their firms profitable by extracting surplus value through extending the working day or increasing the pace and efficiency of production.
It's easy to understand this concept on production lines. If you're bottling a hundred curry pasta sauce jars an hour, sixty of those might be necessary to pay your wages and other overheads, but every jar after that is surplus. Some goes directly into a capitalist's pockets, but much of it is reinvested into production to keep firms competitive. Socialists call this phenomenon 'exploitation.'"
- Bhaskar Sunkara, from The Socialist Manifesto: The Case for Radical Politics in an Era of Extreme Inequality
